In recent years, a hybrid vehicle is focused on as one type of vehicle of environment-friendly automobiles. The hybrid vehicle generally provides an internal combustion engine (hereinafter simply referred to as engine) powered by fuel such as gasoline or the like, and an electric motor (hereinafter simply referred to as motor) powered by electric power from a battery.
There is already developed an above mentioned type of a hybrid vehicle having mounted thereon a vehicle control apparatus comprising, for example, an engine, a motor, a torque converter and an automatic transmission mechanism connected in series, additionally provided with the battery and a control unit (for example, refer to Patent Document 1).
The torque converter is a fluid-type converter using circulating hydraulic oil, and is adapted to transmit output torque of the motor to the automatic transmission mechanism. The torque converter is adapted to amplify a torque inputted thereto and output the amplified torque by operation of the circulating hydraulic oil. Further, the torque converter is provided with a lock-up clutch for connecting and disconnecting between the motor and the automatic transmission mechanism.
The lock-up clutch is adapted to disconnect the automatic transmission mechanism from the motor, when the engine or the motor is over-loaded while the vehicle is running, with the result that the torque converter starts slipping to amplify the output of the engine or the motor.
On the other hand, the lock-up clutch is adapted to lock up the motor and the automatic transmission mechanism by connecting each other, when the engine or the motor is slightly loaded, with the result that the torque converter stops slipping, thereby improving transmitting efficiency of the driving force transmission system and gasoline mileage of the vehicle.
The control unit determines whether or not a crossing point of a vehicle speed and an accelerator opening degree at each moment is within a region in which the lock-up clutch should be connected (hereinafter referred to as lock-up region) in a vehicle-speed-accelerator-opening-degree map, by referencing the preliminarily set vehicle-speed-accelerator-opening-degree map. When the control unit determines that the crossing point of a vehicle speed and an accelerator opening degree at each moment exists in the lock-up region, the control unit connects the lock-up clutch.
The battery is adapted to charge electric power generated by the motor or to discharge electric power for driving the motor in accordance with operating condition of the vehicle. The control unit is adapted to calculate a residual electrical quantity of the battery (hereinafter referred to as SOC (state of charge)) based on an accumulating value of charge current and discharge current of the battery.
The control unit is adapted to expand the lock-up region of the vehicle-speed-accelerator-opening-degree map, when the SOC of the battery is large and the motor can generate large driving force. Thus, the control unit is adapted to raise a frequency of the lock-up clutch being connected in the torque converter, thereby improving transmitting efficiency of the driving force transmitting system and gasoline mileage of the vehicle. Additionally, the control unit is adapted to limit input/output of the battery and use of the motor, in order to protect the battery, when SOC of the battery is excessively large.